I'm having difficulty understanding how larger grains would 'trap' sparks better, and why that would be desireable. That just doesn't seem to fit. There will be larger 'holes' between the grains with larger granules, but the physical dimensions of the burning spark material are much smaller, so the 'holes' between the finer granules are big enough. Plus, the smaller granules have more surface area to ignite. Finally, 'trapping' the spark material isn't the goal - it's subjecting the granule surface to high temperature, which is accomplished by landing the burning material on the grain surface.larger grains would trap the sparks better
A possibility,doesnt look like that frizzen throws a whole lot of spark. May reharden it some day.